Recent studies reveal unexpected complexity in the myosin content of cells in nearly all avian muscles. The primary objective of these studies is to determine the developmental basis for such heterogeneity. This diversity of expression may reflect either intrinsic differences within discrete myogenic cell lineages or extrinsic modulation of a single myogenic lineage or a combination of both. To understand the nature of fiber diversification during development, two experimental approaches will be used. The first is an analysis of the pattern of fiber diversification within a variety of muscle groups during development. This includes a qualitative and quantitative assessment of myosin heavy chains and myosin light chains of distinct muscle groups and individual fibers using monoclonal antibodies, two dimensional gel electrophoresis, Western blots, peptide maps and immunohistochemistry of cryostat sections. The second approach involves the experimental manipulation of early myogenic cells, either by direct electrical stimulation of myotubes formed from myoblasts in cell culture or by surgical modification of the pattern of innervation of developing muscles followed by the analysis listed above. Specific aims include: 1) Determination of the number of myosin isoforms and the distribution of the antibody determinants on these isoforms with respect to the structural domain of the myosin molecule. 2) Determination of whether the heterogeneity of the cell types seen in the adult is derived from more than one muscle cell type in the embryo. 3) Determination of whether electrically induced contractions of myotubes in cell culture alter myosin isoform transition and synthesis. 4) Determination of the role of innervation in the establishment of the earliest pattern of muscle fiber formation using spinal cord rearrangement and limb reversal operated embryos.